In the fields of process parameter measurement and process control, it is frequently desirable to employ and/or locate sensors at stages or steps of a processing chain for the purpose of measuring process variables at that particular stage or step. Many different types of sensors may be utilized to measure many different process control variables such as temperature and pressure. Some examples of common sensors include strain sensors for measuring torque and pressure, and mass flow controllers which are used to measure and control the flow of gas through a tube. In the electronics industry, such mass flow controllers are often used in the fields of semiconductor processing and fabrication where it is necessary to accurately and precisely measure the temperature and pressure of a gas or other chemical agent at individual points in the processing path. An example of a sensor used with a mass flow controller can be found in U.S. Pat. No. 4,464,932 issued to MKS Instruments, Inc. of Andover, Mass., the present assignee. This patent illustrates an improved mass flow controller which incorporates a number of sensors for measuring the temperature and flow of a gas.
While such measurement techniques are well known, it is also well known that ambient temperature may affect the sensitivity and gain of such sensors, and may further superimpose an undesirable offset on the output of such sensors. Therefore, when designing electronic circuitry that is used to interface a sensor for the purpose of measuring some physical parameter with that sensor, it has been a common practice to correct for the effects of temperature on the sensor, or the effects of temperature on the associated sensor circuitry, by adding elements with known repeatable temperature coefficients in such a way as to null the thermal effects on the sensor and its circuitry.
An example of this practice has been to include in the sensor signal output path a temperature compensation element such as a resistor with known and repeatable temperature coefficients (like a thermistor), incorporated in such a way as to change the gain of the circuit, or the gain of the output of the sensor due to changes in ambient temperature, in a direction equal and opposite to those changes due to temperature. This frequently requires mounting the temperature compensation element in such a manner as to insure good thermal conductivity to the sensor. It is also a common practice, often employed in conjunction with the above-known gain compensation circuitry, to mount a second temperature compensation element in a similar manner and wired in a similar way such that the second temperature compensation element will counterbalance the change to any offset (including zero offset) of the sensor circuitry which also occurs as a result of changing temperature.
The prime deficiency in the above-noted circuit topology is that two temperature compensation elements must be used, and matched, and then mounted in such a way that they receive similar temperature information in order to provide compensation to the output of the sensor in a coordinated manner, increasing circuit design time and cost and decreasing operating flexibility. Accordingly, it has been determined that the need exists for a circuit design which allows for a single temperature compensation element to compensate for the undesirable thermal effects imposed on both the offset and signal gain of a sensor and its associated circuitry. Such a single temperature compensation element embodiment may, therefore, offer improved temperature compensation with reduced circuit complexity and production costs.